Rocking the AC

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http://physics.ucsd.edu/do-the-math/2012/09/rocking-the-ac/

When it comes up in casual conversation that I do not generally heat or cool my house, people either move to another seat or look at me with some mixture of admiration and disbelief. When non-Californians then find out that I live in San Diego, they might huff or spew, which often involves some embarrassing projectile escaping their mouth. But the locals are more consistently impressed — more so by my forsaking heat than AC (San Diego has very mild summers by U.S. standards). This summer, I turned on the AC for the first time since we bought the house three years ago. All in the name of science! I was blown away. Here is what I learned.

A Happy Convergence

I occasionally embark on a campaign to track temperatures throughout my house in order to understand how it behaves thermally in response to fluctuations in sunlight and external temperature. I keep telling myself that I’m establishing a baseline against which to compare the results of a hypothetical insulation upgrade/replacement. So far, I keep getting baseline data.

Most telling are campaigns when the external temperature is extreme in some way. The forecast showed a hot week was coming up, so I deployed the Thermochrons for an 8.5 day mission, set up to sample temperatures to 0.06°C precision every three minutes (and roughly synchronized with each other).

As it happened, the first day into the campaign was unusually hot and muggy for San Diego (high 30 °C, or 86 °F, 62% humidity). And we had invited 2500 W over for a surprise party (non-physicists might instead say 25 people). Okay, so Thermochron campaign, hot and muggy, loads of people coming over…if there was ever a right time to fire up our AC, this was it!

We Have Liftoff!

I set the thermostat to 75°F (24°C) flipped the switch at 4PM, and immediately rushed to TED to see what he thought of our unfamiliar visitor. I had never seen anything like it. TED was screaming that we had over five kilowatts running! Previously, the highest rate of electricity consumption I had ever seen in our household was in the neighborhood of 3 kW when both the microwave and toaster oven were in simultaneous operation.

I rushed outside to watch the synthetic dial “spin” on my electricity meter. Usually, I have to wait a full minute to see one block change, but now blocks were coming and going faster than once per second. Night and day difference!

100 W at a time, our guests arrived and were appreciative of the cool — though those who knew me well admitted to being surprised. The air conditioner ran constantly for 4.5 hours, and I began to wonder if the system had enough oomph to overcome the party load. But it finally started cycling, spending about 45% of the time in the on-state over the next five hours.

I clocked 32.6 kWh that day. This may not sound like a lot. In fact, it’s pretty close to the U.S. average daily household electricity consumption. But for me, it was out of this world. For perspective, over the previous 600 days, our typical daily expenditure (median) was 1.64 kWh (average 1.82, with 0.79 standard deviation). The minimum was 0.85, and the max soared to 6.3. 32.6 kWh beat our previous maximum by a factor of five, and our typical usage by a factor of 20! Welcome to America, my friend.

Are We There Yet?

As much as I learned from this experience, the day of the “test” could hardly be called typical.